1. Field of the Invention
The invention is directed to a method for producing a foamed slag on austenitic stainless melts in an electric arc furnace. A mixture of metal oxides and carbon is introduced into the furnace, the metal oxide is reduced by the carbon below the slag at the metal/slag interface, and the resulting gases bring about the foaming of the slag by bubble formation.
2. Related Art
In the operation of electric arc furnaces, the charge, i.e., primarily scrap and alloys, is melted by the electric arcs of electrodes projecting downward into the furnace vessel.
The slag in the foamed state carries out a protective function in addition to its primary function of removing unwanted components from the melt.
In this state, the slag envelopes the space between the ends of the electrodes and the metal surface and protects a refractory lining of the furnace from radiant energy of the electric arc. Since the foamed slag is a poor conductor of heat, the radiation of the electric arc against the wall of the electric furnace is sharply reduced, and the introduction of energy into the metal melt is accordingly improved.
In the case of nonstainless steels or steels with a low chromium content, the foamed slag is achieved by injecting carbon and oxygen simultaneously.
In melts with high chromium content, the injected carbon reacts principally as a reducing agent of the chromium oxide. Due to the very low iron oxide content in the slag, the desired degree of foaming cannot be achieved.
According to EP 0 829 545 B1, an improvement is achieved in that a powder comprising a metal oxide, either zinc oxide or lead oxide, and carbon is introduced into the slag. The oxide contained in the powder is reduced by reaction with the carbon, and bubbles are formed by the occurring carbon monoxide for foaming. The large surface area of the powder results in very powerful, locally limited reactions.
It is known from DE 103 23 505 to charge preforms such as pellets or briquets instead of powder to achieve a very controlled, uniform and gradual reaction.
However, problems occur whenever a slag with concentrations of chromium oxide greater than 10% is formed during the melting of the solid material in the electric arc furnace. It has not been possible thus far to liquefy and foam slags of this kind to the desired extent because of their composition.